A technique is known, wherein the reduction control is executed for NOx absorbed in an absorption reduction type NOx catalyst (hereinafter simply referred to as “NOx catalyst” as well), and then it is judged that the NOx catalyst is deteriorated if the NOx concentration, which is detected by a NOx sensor disposed on the downstream side from the NOx catalyst, is not less than a predetermined concentration at a point in time at which an estimated value of the absorption amount of NOx in the NOx catalyst arrives at a reference value (see, for example, Patent Document 1).
However, it is necessary to wait until a large amount of NOx is absorbed and stored in the NOx catalyst, and a long time is required to judge the deterioration or degradation of the NOx catalyst. For this reason, if the NOx catalyst is deteriorated or degraded, it is feared that NOx may outflow during a period until the deterioration judgment is completed.